James N. Piper

Refraction and scattering into a sandy ocean sediment in the 30-40-kHz band

To observe sound penetration into a sandy sediment, a buried acoustic receiving array was insonified by a wide band sound source carried by a remotely operated vehicle. A slanting array design was used to avoid scattering artifacts. This design overcame possible problems in previous experiments, in which scattering artifacts from the array structure could be mistaken for a propagating wave. The experiments took place in a sandy sediment off the West coast of Florida, as part of the sediment acoustics experiment, which is a multidisciplinary effort to study sediment acoustics. A coherent angle, speed, and height estimation process searched through a four-dimensional search space, of source height and elevation angle, wave speed, and propagation delay to find spherical acoustic wave fronts. Three main categories of waves were found: first refracted, dominant nonrefracted and evanescent. Later acoustic arrivals, a fourth category, remain to be analyzed. Their relative intensities effectively characterize the sediment penetrating acoustic energy. The acoustic sound pressure level of penetrating waves below the critical grazing angle was found to be greater than expected for a flat interface.

Quantifying the effects of roughness scattering on reflection loss measurements

Seafloor reflection loss and roughness measurements were taken at the Experimental Validation of Acoustic Modeling Techniques experiment in 2006. The magnitude and phase of the reflection loss was measured at frequencies from 5 to 80 kHz and grazing angles from 7° to 77°. Approximately 1500 samples were taken for each angle. The roughness was measured with a laser profiler. Geoacoustic parameters such as water and sediment sound speed and density were measured concurrently. The reflection loss data were compared with three models: A flat interface elastic model based on geoacoustic measurements; a flat interface poro-elastic model based on the Biot/Stoll model; and a rough interface model based on the measured interface roughness power spectrum. The data were most consistent with the poro-elastic model including scattering. The elastic model consistently predicted values for the reflection loss which were higher than measured. The data exhibited more variability than the model due to layering and fluctuations in the propagating medium.

Geoacoustic inversion of short range source data using a plane wave reflection coefficient approach

Acoustic time series data were collected in a shallow, hard bottom lake environment located in central Texas using both short range (2 m) implosive data, obtained with the source and a single hydrophone located near mid-depth in the waveguide, along with longer range implosive and explosive data from a near surface source to a bottom mounted hydrophone. Matched field inversions using simulated annealing were performed with a ray trace plus complex plane wave reflection coefficient forward propagation model that was validated in previous work. Isolating bottom interacting paths to perform the inversions is shown to be essential to reduce parameter uncertainties in the hard bottom environment and enables a systematic approach to the inversions which establishes the number of layers needed to represent the lake environment. Measured transmission loss data from a towed source were compared through a RMS error analysis to modeled transmission loss, constructed with the parameters from inversions of data from several source types, to further establish the validity of the inversion approach for this environment. Geoacoustic parameters obtained by inversions of short range, low frequency impulsive data are used to predict transmission loss at longer ranges and higher frequencies. The range dependence of the global minimum is discussed.

Finite element modeling of propagation and reverberation shallow water waveguide with a variable environment

Shallow water waveguides can exhibit environmental variability in the water column, on the bottom interface and in the sediment. Finite element models provide a method to capture the effects of this variability since every element can be described by a different sound speed and density. However, fully three-dimensional finite element models are often computationally inaccessible due to extreme memory requirements. In this study, a longitudinally invariant finite element model is used to predict the reverberation from a shallow water waveguide described by environmental measurements at the Target and Reverberation Experiment 2013 conducted off the coast of the Florida panhandle. Longitudinally invariant models retain all of the fidelity of a three-dimensional model with the requirement that one geometric dimension must be invariant. Therefore, it is an ideal model for wedges and ridges. In this case, the longitudinally invariant direction describes the sand ripples. The reverberation with and without varia...

Sea floor roughness measured by a laser profiler on a ROV

Improvements to a method of seafloor roughness measurement using low-power lasers and a video recorder mounted on a remotely operated vehicle (ROV) are presented. The platform is a Saab Seaeye Falcon observation class ROV. Six green laser beams are spread into illuminated planes using gratings or cylindrical lenses. The intersection of the laser planes with the ocean bottom was imaged on a high-definition digital video recorder at an oblique angle. After correcting for aberrations due to refraction between air and water and other imperfections, the three-dimensional coordinates of the laser profiles relative to the video recorder are computed for each video frame. This is a low-cost device, since the laser modules are commercial line generator modules of moderate power output. Green lasers were chosen for their low attenuation in seawater. The power output of each laser module was chosen to be within the Class 3R designation to avoid the permitting issues of high power lasers, although high power units are being developed. The lasers are housed within an optically clear acrylic tube. In addition to the laser modules, a light-emitting diode (LED) white light source is also deployed within the laser housing to provide background illumination of the seabed around the laser lines. The digital video recorder is a high definition consumer-grade device, housed in a separate tube with a clear acrylic window. Power is provided by the ROV. The signal and controls are carried by one shielded twisted pair cable through the ROV umbilical to the shipboard control unit. Auxiliary data include roll, pitch and heading information are simultaneously recorded. Ultra short baseline (USBL) acoustic positioning information from the ROV is also recorded. In previous experiments, the laser planes were arranged in parallel, but in the current configuration some laser planes are rotated to intersect with the others. The intersections present additional difficulties for the automatic tracing and identifying software, but they provide a number of advantages in calibration and position tracking.

Modeling of reflection coefficient fluctuation from measured seafloor roughness

Bottom roughness measurements were made in conjunction with acoustic reflection measurements to determine the effects of incoherent scattering on the mean value and fluctuations of the reflection coefficient over a wide range of angles. A number of methods are available for modeling the fluctuation of the received signal, hence the reflection coefficient. Numerical methods, such as the finite element method, are certainly possible given the short distances involved in this experiment. However, for better insight into the physics, analytical methods are preferred. In this study, the analysis is based on the second moment of pressure amplitude, as formulated by Tolstoy and Clay.

Small scale seafloor roughness measurement using a ROV

A method for seafloor roughness measurement was devised for a remotely operated vehicle (ROV), consisting of lasers and a video camera. It was designed to measure small-scale roughness in the range of spatial wavelengths from 0.01 to 1 m. This range of wavelengths is expected to be relevant to acoustic bottom backscattering in the frequency range roughly from 1 to 100 kHz, depending on grazing angle and orientation.

Measurements of the Bottom Loss Magnitude and Phase from 5 to 50 kHz and 10 to 77 degrees grazing at the Experimental Validation of Acoustic modeling techniques (EVA) Sea Test

The value of the specular bottom loss is an important parameter for modeling the propagation of acoustic waves in littoral areas. In shallow waters, even short range propagation can have many interactions with the ocean bottom. Bottom loss is affected by many envorinmental parameters including the reflection coefficient, which is dependent on the sediment type, and scattering from the water/sediment interface. The phase and magnitude of the reflection coefficient from 5 to 50 kHz and 10 to 77 degrees grazing from a rough sand/water interface were taken as part of the Experimental Validation of Acoustic modeling techniques (EVA). The experiment was conducted off the coast of Isola dElba in October 2006. The measurements were corrected for beam pattern and rough interface scattering effects and compared with two current models describing acoustic interaction with ocean sediments, the elastic model and the Effective Density Fluid Model (EDFM) [1]. Both models are corrected for spherical wave effects using the experimental geometry.

High frequency signal amplitude fluctuations in shallow water

Amplitude fluctuations were measured in a high‐frequency (5–50 kHz) experiment over short ranges in 10 meters of water on the north shore of Isola dElba, Italy. The fluctuations, as a function of range and frequency, are compared with estimates using the empirical refractive index wave‐number spectrum put forward by Medwin [J. Acoust. Soc. Am. 56, 4 (1974)] for the upper ocean region. The spectrum is governed by the distance from the nearest boundary and the standard deviation of the refractive index. The results illustrate the ubiquity of refractive index fluctuations and the elegance of Medwin’s model. [Work supported by Office of Naval Research, Ocean Acoustics.]

Spatial variation of seabed acoustic bulk properties

The seabed is modeled as a poro-elastic medium with a rough interface. The spatial variation of bulk properties, along with the interface roughness, are important contributors to the acoustic scattering strength of the seabed. Their effects are often indistinguishable. While roughness may be measured directly, the variability in the bulk properties is more difficult to obtain. In a recent experiment over a sandy seabed off Panama City, FL, known as the target and reverberation experiment of 2013 (TREX13), the seabed roughness and the normal acoustic reflection loss were simultaneously measured using a laser profiler and a short range acoustic sounder deployed aboard a remotely operated vehicle (ROV). Using the measured roughness statistics, the fluctuations in acoustic reflection loss due to roughness were estimated. Subtracting the roughness contribution from the total measured reflection fluctuations, the component due to bulk property changes was estimated, from which the fluctuation in the bulk proper...